Receptor-associated tyrosine kinases, such as Jak2 and Src, serve as proximal mediators of ligand binding. Abundant data indicate that both of these kinases significantly contribute to the pathogenesis of breast cancer, and yet the mechanisms leading to their activation has remained uncertain. Our research, which has focused on the breast cancer-relevant receptor for prolactin (PRLr), has revealed that the peptidly prolyl isomerase (PPI), cyclophilin A (CypA) is required for the activation of these kinases. These findings would indicate that PPI activity of CypA is involved in a conformational restructuring of this receptor-kinase complex that contributes to its activation. Additional studies have revealed that the formation of a multimeric complex between the PRLr, Jak2, Src, and CypA is necessary of ligand-induced activation of the PRLr-associated signaling complex. At a translational level, these results have been further corroborated by our evaluation of a CypA knockout model and the successful use in breast cancer models of the PPI inhibitors cyclosporine A (CsA) and NIM811 both in vitro and in vivo. Taken together, our findings lead us to hypothesize that the intermolecular interactions between these proteins and the PPI activity of CypA result in the triggering of Jak2 and Src following ligand engagement, and that such events are highly relevant to the pathogenesis of breast cancer. This hypothesis will be tested in three specific aims, as follows: First, the functional role of protein- protein interactions within the PRLr/Jak2/Src/CypA will be evaluated using mutagenic, overexpression, and knockdown approaches within breast cancer cells. Second, in vitro kinase and soluble and solid state NMR spectroscopy will be used to assess conformer status with the PRLr/Jak2/Src complex as regulated by the PPI activity of CypA. Third, both gain- and loss-of-function approaches will be used in genetic and xenograft-based murine models to assess the role of protein interactions and PPI function within the PRLr complex during the pathogenesis of mammary cancer. The studies proposes are highly significant in that the will provide a molecular foundation for our understanding of the structure/function relationships during receptor-Jak2/Src activation as modulated by the PPI activity CypA and translate these discoveries through cellular to mouse models of breast cancer.